Quoting the Crisis

Specifically, the Oliver Wyman study assumes that every dollar disallowed in pure proprietary trading by banks will necessarily disappear from the market. But if money can still be made (without subsidies), the same trading should continue in another form. For example, the bank could spin off the trading activity and associated capital at a fair market price. Alternatively, the relevant trader – with valuable skills and experience – can raise outside capital and continue doing an equivalent version of his or her job. Now, however, these traders will bear more of their own downside risks.

If it turns out that the previous form or extent of trading only existed because of the implicit government subsidies, then we should not mourn its end.